Process and apparatus for purifying gases



Nov. 22, 1960 P, J, HARlNGHUlzEN 2,960,836

PROCESS AND APPARATUS FOR PURIFYING GASES Filed Sepl.. 27, 1955 3Sheets-Sheet 1 il mm www Nov. 22, 1960 P. J. HARINGHUIZEN 2,960,836

Paocass AND APPARATUS FOR PURIFYING GAsEs A Filed Sept. 27, 1955 3Sheets-Sheet 2 FIG. 6

FIG. 7

Nov. 22, 196() P. J. HARINGHUIZEN 2,960,836

PRocEss AND APPARATUS FOR PURIFYING GAsEs Filed Sept. 27, 1955 3Sheets-Sheet 3 HJM tpm?! Mfr) ATTORNEYAl United States Patenti PROCESSAND APPARATUS FOR PURIFYING GASES Pieter J. Haringnnzen. Geleen,Netherlands, assigner to Stamicarbon N.V., Heerlen, Netherlands FiledSept. 27, 1955, Ser. No. 537,070

Ciaims priority, application Netherlands Oct. 1, 1954 16 Claims. (Cl.62-13) The invention relates to the purification of gases by the removalof gaseous impurities and is particularly but not exclusively concernedwith the purification of technical hydrogen, e.g., such hydrogen as canbe obtained in the water gas reaction, by cooling to a temperature below65 K. to deposit in the solid form residual impurities such as nitrogen,carbon monoxide and argon. The invention will consequently be moreparticularly described in the latter application, which will be ofspecial importance in connection with the distillation of hydrogen.

The distillation of hydrogen on a technical scale is employed as a meansfor the separation of hydrogen into its isotopes and the isolation ofdeuterium. Ordinary `hydrogen, however, contains no more than 0.015% ofdeuterium. This implies that if considerable quantities of deuterium areto be produced, very large quantities of hydrogen have to be distilled.Consequently, a deuterium plant is only an economical proposition if thehydrogen, after being freed of deuterium, can be immediately used forother purposes, e.g., for the synthesis of ammonia. A deuterium plantconnected to an ammonia synthesis plant is described in Chem. Eng.Progr. 50, 5; 227-228. In order that the hydrogen can be cooled down tothe distillation temperature and distilled, it is necessary for theother gases present in the hydrogen, such as nitrogen and carbonmonoxide to be removed very thoroughly. In the following only theremoval of nitrogen will be dealt with. However, the invention is notrestricted to the removal of nitrogen and other gases such as argon,carbon monoxide and oxygen may also be removed in the same manner,simultaneously with the nitrogen.

As a matter of course, the major portion of the nitrogen may be removedfrom the hydrogen by cooling the latter in heat-exchangers to atemperature lower than the condensation temperature but higher than themelting point (63 K.) of nitrogen. Cooling below the melting point ofnitrogen, before the major portion thereof has been removed, would giverise to obstructions by solid nitrogen.

After removal of the liquid nitrogen, the hydrogen, whichdepending onits pressure-may still contain 2-3% of nitrogen, must be cooled downfurther, but during this operation solid nitrogen is deposited.

In the following the use of heat regenerators only will be discussedthough it will be understood that reversing heat-exchangers may beequally well used.

The present invention is directed to depositing the impurities in a gaswhile cooling in a zone and subsequently ushing the zone in the oppositedirection with a diiferent gas in order to evaporate the depositedimpurity, the temperature being controlled so that in the zone theaverage Vtemperature is lower during the deposition than it is duringthe evaporation. n

Referring to the drawings, Figures 1-4 diagrammati -cally illustrate theflow in a battery of four regenerators operating according to theinvention in a four-period cycle. v-

Figures 5, 6 and 7 indicate three different temperature patternsobtained in each of the four regenerators by three different methods ofcontrol.

Figure 8 is a diagrammatic flow sheet.

It has been found that the maximum vapour tension of the nitrogenincreases considerably with the hydrogen pressure and to such adegreethat at certain temperatures the maximum percentage of nitrogenvapour in hydrogen may even be higher in hydrogen of higher pressurethan in hydrogen of lower pressure.

Now we have found how the hydrogen can be freed of nitrogen by freezingthe latter out in heat-regenerators, notwithstanding the way in whichthe maximum vapour tension of nitrogen in hydrogen changes unfavourablywith the hydrogen pressure as before referred to.

In contrast to the past procedures, in the process according to theinvention for the purification of a gas by cooling and deposition ofimpurity by condensation to the solid state employing a heat-regeneratoror reversing heat-exchanger, the temperature is controlled so that thetemperature at any place in the regenerator or exchanger is on averagelower during the deposition than the average temperature at that placeduring the evaporation.

In carrying out the process according to the invention, the regeneratoror exchanger need not be alternately traversed by a gas of higher and agas of lower pressure, but the pressure of the flushing gas used forremoving impurity, e.g., nitrogen, is preferably kept substantiallyequal to the pressure under which the gas to be purified,

e.g., the hydrogen, is passed through the apparatus `for cooling withsimultaneous deposition of the impurity. This offers considerableadvantages, because in this way pressure surgesrin the equipment can beprevented when regenerators are being exchanged. In a fully enclosedapparatus, these pressure surges would give rise to considerablediiculties.

The process according to the invention makes use of heat-regeneratorsprovided with at least one pipe system which is to serve for passing asecond gas in the opposite direction or of reversing heat-exchangers oflarge heat capacity, whereas the cold gases need not be of differenttemperatures.

During the period in which the impurities are deposited and/or duringthe flushing period, this large heat capacity is used for varying thetemperature at any given point of the regenerator by altering the ratioof the gas quantities which during said period are exchanging heat witheach other. According to the invention this is effected in such a waythat during a period in which the impurities are being deposited thetemperature at any given point of the regenerator will iirst decreasegradually and after that rise again and/or during the period when theimpurities are being dissolved said temperature will first rise andsubsequently fall.

The gas quantities that are exchanging heat with each other arepreferably so chosen that the total heat capacity of each of said gases,considered over the whole duration of a passage period, will be thesame. When switching over, the temperature at any given point of theregenerator will then be equal again to the temperature at the beginningof the period. If not, the temperatures will, at the moment of switchingover, show a slight tiuctuation in which connection it is to be remarkedthat, if the process is to be carried out as a continuous operation,measures have to be taken to ensure that the temperature at any givenpoint will, after completion of the whole cycle, be equal again to thetemperature at the beginning of the cycle. y In carrying out theinvention we prefer to employ api Patented Nov.22, 196mparamscomprisingfour regenerators (or exchangers of suiciently large heatcapacity) each having a passage, packed with filling material, for thegas to be cooled and purified and, subsequently, for the passage of theflushing gas, and comprising also at least two independent pipe systemsthrough each of which gas can be conducted in heat-exchange-relationwith theegas flowing through the` said passage.

With this apparatus, purification according toY the invention mayproceedv continuously in acycle of four periods. Ineachoftheseperiodscooling with deposition of impurity takes place intwoofthe regenerators which are temporarily connected in parallel with thesupply of the gas to be purilied and'flushingitakes place in the othertwo regenerators which are temporarily connectedl in parallel with. a.supply of ushing gas, the parallel pairing of the regenerators changingfrom period to period but so that there. are two consecutive depositionperiods and two consecutive flushing periods in each cycle for eachregenerator. This maybe achieved by connect-A ing the separate pipe`systems of each regenerator into two independent systems common to thefour regenera-y tors, one associated with a source of supply of a gascolder than the gas to be purified and the other with a gas warmer thanthe flushing gas and directing the ow from such sources of supply duringthe four periods so that in each regenerator the ow of gas to be cooledand purified always takes place in countercurrent and heat exchangingrelation with a flow of the colder gas and the flow of flushing gasalways takes place in countercurrent and heat exchanging relation with aow of the warmer gas. By controlling the amounts of gas forming therespective currents. through the regenerators the result may beachievedthat a tiuctuation of temperature takes place in each regeneratorVduring. deposition or tiushing or both such that the temperature at any.place in that regenerator is on average lower during the deposition thanthe average temperature at that place during the evaporation, while thetemperature at any givenpoint of the regeneratoris about the same at thecommencement as at the termination of the deposition periods and also ofthe ilushing periods.

The manner in which the gas currents may be controlled to obtain thedesired temperature effects will best be understood from the exampleswhich will presently be referred to with reference to the accompanyingdrawings, but it may here be explained that, if for example, in a givenperiod of a four period cycle as above referred to the gas to be puriedis passed in equal amounts through two regenerators, then by feeding oneof such regenerators with a higher proportion than thesecond of thecountercurrent colder gas the temperature in the said one regeneratormay be made to decrease while the temperature in the second rises. Nowif during the next period the said one regenerator is to pass through a.second deposition period its temperature will require to be Yincreasedif the temperature obtaining at the commencement of the deposition is tobe restored. The required increase may be obtained by feeding the saidone regenerator during the said next period with a proportion of thecountercurrent colder gas corresponding to that which in the previousperiod passed to the said second regenerator provided that in theoperation of the four regenerators according to the cycle outlined abovethe quantities of gas which in any period are in heat exchangecountercurrent flow in a pair of parallel connected regenerators aresuch that the heat capacities per degree, i.e., the products of thespecific heat and the quantities, are Vpractically equal to one another.When through the whole temperature Vrange covered by the regenerator thespecific heat of the one current isconsiderably higher than that of theother current, the two currents must differ in the same proportion. Inorder to attain that the amount of gas to be cooled may yet be equal tovthe amounts of gas to -be heated,ja separate cooling will have to beapplied in that case. If the difference in specific heat is onlynoticeable on the cold side, this etect can be eliminated by applyingthe socalled Iunbalance liow as is customary in reversing heatexchangers(cf. e.g., Chem. Eng. Progr. 43, 2; 69-73 (1947).

As previously stated the invention is more particularly intended forapplication to the purification of technical hydrogen and particularlyin hydrogen distillation and in that application the cold and N2-freehydrogen expanded to a low pressure (e.g., 1.3 ats.) obtained from thedistillation process, may, upon heat exchange, be compressed to a highpressure (eg. 100 ats.) and restored to the temperatureV level at the.entrance of the regenerators for supply through one of the pipesystemsin countercurrent to the liushing gas, and after traversing theregenerators the said hydrogen, whilst delivering its cold, may beexpanded to a pressure equivalent to the pressure of the hydrogen to bepurified, say 12 ats. and then returned to the regenerators as lushinghydrogen.

Referring to Fig. 1, the four regenerators 1, 2, 3 and 4 each comprisesa space a packed with lilling material, a pipe system b. anda pipesystem c each of which is adapted to conduct gas in countercurrent flowand heat exchanging relation with respect to gas for the time beingflowing through spacek a. The spaces aof the four regenerators haveconnection on the left hand side with a source 0f hydrogentobewpuriiiedand on the right hand side with a source of-1iushing hydrogen. The pipesystems bof-the four regenerators have connectionv with a source Vofexpanded cold hydrogen, e.g., a distillation apparatus, and the pipesystems c of` all the regenerators` have `connection with aY source ofhigh pressure hydrogen.

In the rst period (Fig. 1)` equal currents ofhydrogen to be cooled andpuriied, indicated by full lines g, are passed through regenerators 1and 2 at a pressure p ot 12 ats. and at atemperature slightly above themelting point of N2, say 65 K. while currents or" expanded cold hydrogenindicated by broken lines e are passed at a pressure. q of say 1.3 ats.through the pipe systems b. of the said regenerators in countercurrentto the hydrogen to be purified. The quantity of expanded cold hydrogenpassing through pipe system b of regenerator 2 is less than the quantitypassing through pipe system b of regenerator 1. This. is indicated inthe iigure by the sign d. (This sign, wherever it appears in Figures lto 4 indicates a reduced current with respect to the parallelcurrentfrom the same source.) In consequence, in the first period,depositionA of nitrogen takes place in both regenerators 1 and 2 butwhereas at every point in regenerator 1 the temperature falls duringthis period, the temperature at everyV point in regenerator 2 rises dueto the decit of cold in this regenerator.

Also in the lirst period, equal iiushing currents of cold hydrogenindicated by full lines h, are passed through regenerators 3 and 4,which are to be regarded as already containing a nitrogen deposit, at apressure p of say l2 ats. while currents of high pressure hydrogen(pressure P of say'lOO ats.), indicated by chain lines are passedthrough the pipe systems c of the said regenerators in countercurrent tothe said flushing hydrogen. As indicated by the sign d in chain line fmore than of the warm high pressure hydrogen passes through regenerator3 in period 1. lIn consequence, evaporation of nitrogen takes place inboth regenerators 3 and 4 during the first period but Whereas thetemperature at every point in regenerator 3 rises during this period,the temperature at every point in regenerator 4 correspondingly fallsdue to the heat deficit therein.

In the second period (Fig. 2), the current of hydro-` gen to be purifiedwhich in the previous period passed Vto regenerator 2 is now switched toregenerator 4, the cleaningrof which was completed inthe previousperiod, deposition continuing in regenerator l. At the .same

time the current of ushing gas which in the previous period passed toregenerator 4 is now switched to regenerator 2 in which deposition wascompleted in the said previous period. In this period thereforedeposition proceeds in regenerators 1 and 4 and liushing in regenerators2 and 3. Furthermore, for the second period, the ow of the high pressureand cold expanded hydrogen is also readjusted. This, inter alia, pipe bof regenerator 1 now receives a reduced amount of the expanded coldhydrogen so that the temperature of this regenerator commences to riseso that by the end of the deposition stage in this regenerator, i.e., atthe end of the second period, the temperature will about be restored toits value at the commencement of the said stage. Likewise, thetemperature of regenerator 3 which rose during the previous period isnow reduced in consequence of a reduced supply of the high pressurehydrogen the major proportion of which is passed to regenerator 2 inwhich ushing is just commencing.

The current control is again adjusted at the commencement of the thirdand fourth period and it will be readily seen from the ow lines that theresult is that over the four periods the temperature in each regeneratoralternately rises and falls. The quantities of hydrogen in the currents(taken collectively) which in each period in each pair of parallellinked regenerators pass in heat exchange relation one to the other aresuch that the heat capacities per degree, i.e., the products of the'spe-- cic heat and the quantities, are substantially equal. Moreoverthe adjustment of the currents from period to period is such that in thesecond period, as compared with the tirst period, then as far as thecharacter and quantity of the hydrogen passing through the regenera-Vgaat current'passing through regenerator 3 smaller than the currentpassing through regenerator 4.

It is also possible to employ a combination of the two methods, e.g.,unequally distributing only the gases to be cooled.

According to another way of carrying out the invention, only one of thecurrents is distributed unequally, whereas the other three are keptconstant. For example, if only the quantity of cold expanded hydrogen isunequally distributed in such a way that in each regenerator thetemperature decreases at the start of the iirst deposition period andincreases in the following period, whereas during the t'wo periods inwhich evaporation takes place the temperature remains const-ant owing tothe equality of the other currents, the condition will yet be satisfiedthat the average temperature at which the deposition is eiected must belower than the temperature at which the deposited impurity is taken upagain. The

- temperature iluctuation obtained by the method is shown' mentionedpossibilities, i.e., to the procedure in whichv tors are concerned,regenerator 4 takes up the place of- 1 regenerator 1, 3 takes up theplace of 4, 2 takes the` place of 3 and 1 takes the place of 2, and thisarrangement is continued through the subsequent period transitions. Theresult is that the rise and fall of the temperature in each regeneratoris more or less regular.

This temperature fluctuation is shown in the graph of Fig. 5 (full linela) in which the temperature of the lling material at one place inregenerator 1 has been plotted on the vertical axis against the time onthe horizontal axis. The numerals I, II, III and IV indicate the foursuccessive periods.

In the periods I and II the hydrogen current to be purified is cooledand nitrogen is deposited and in periodsl III and IV the depositednitrogen is taken up by a ushing current of hydrogen of preferably thesame pressure as the hydrogen to be purified. The temperature variationof the gases at the place in question in the regenerator is shown bydotted line tg (hydrogen to be puried) and dash line th (iiushinghydrogen). In the periods III and IV the average temperature of the gas,and hence also the maximum vapour tension of the N2, is higher than inthe periodsvI and II in which the N2 is deposited. This makes itpossible for a larger amount of N2 to be evaporated in the same amountof hydrogen than has been deposited during the previous period.

fore possible to remove all of the N2 deposited.

In a well designed heat-regenerator the difference betweenV thetemperatures of the gas and the filling material is only small, e.g., 1or less.

In anv alternative way of operating the regenerators, the sametemperature uctuation effect may be obtained' by replacing the unequaldistribution of the gas currents passing through the pipe systems b andc by the unequal distribution of the hydrogen current to be puriied andythe backward ushing current. y v f,

To obtain the same temperature effect in the situation illustrated inFig. l, the current of hydrogen to be puried in regenerato-r 1 wouldhave to be smaller than the, current supplied to regenerator 2 and theilushing gas which it is dissolved `will be largest. v-

pressure of these gases.

the quantities of the gases passing through the pipe systems b and c aredistributed unequally, For in this case, the difference between theaverage temperature of the gas in whichthe-impurity is deposited and ofthe gasiu,

Howlarge the temperature differences have lto be inorder to permit theimpurity to be completely removed depends on the construction of theregenerators, the velocity at which the gases are passed through and onthe An average temperature difference of 3-5a will be sufficient in mostcases. Referring to Figure 8, the numerals 1, 2, 3 and 4 design-ate thefour heat exchanging regenerators. a designates the free space of theregenerators; b the tubes for the cold gas; c the pipe systems for theWarm gas at high pressure. 'I'he source of technical hydrogen isdesignated as 11. The hydrogen is led through tubes 12 to a heatlexchanger Where the gas is cooled down to a temperature of about thefreezing point of nitrogen. In 13 liquefied Y, flushing the regenerators1, 2, 3 and 4. The hydrogern is fed to the exchanging valves 16, 17, 18and 19 through., line 1'5.V In the regenerators the hydrogen is cooled?down and the contaminations are frozen out in solid form.A The puriiiedcold hydrogen is fed to the plant (e.g., a,

-. hydrogen distillation unit) through line 21. Expanded hydrogen is fedback through line 31 via vlalves 36, 37 38 and 39 to the tubes b of theregenerators, delivers its; cold and ows through line 41 and the heatexchanger to the pump 42, where it is compressed to a high pressure,cooled down below room temperature in heat exchanger 43 and fed backthrough the exchanger 5 and tubes 44, viathe valves 46, 47, 4S and 49 tothe tube systems c of the regenerators 1, 2, 3 and 4. Here the highpressure gas ows countercurrent wit-h the iiushing gas.. The cooled gasis fed to the plant through line 51 heat exchanger 5:back to thehydrogen source together.

with the nitrogen from tank 13.

The situation of Figures 1, 2, 3 and 4 is as follows: drogen to bepurified andthe. iiusl'iinghydrogen,` should situation valves 4l. a l?.Ab bV b ,a. a JV poc c c c o pa a b b a b z av b po c c o c o pa c b b aa, av a IJA b c c o po o no` c c `b a a, b a b bl a c c po c` po c c o.

a means: in valves 16-19 and 26-29 the passage from the lett to theregenerator is open,

b the passage from the right is open. For valves 36-39 and 40-49 c meansclosed, o opened and po partly opened.

In .the drawing in Figure-8 the situationof Figure l is shown.

The way in whichv the various hydrogen currents for carrying out theillustrated process may be obtained has already been referred to. i

In purifying hydrogen it will be clear that the process according to theinvention cannot be employed up to the distillation temperature of thehydrogen, because the gases passing through the regenerator space cannotbe cooled below the boiling point of the hydrogen correspondlng withthis pressure. Preferably, these gases should not be cooled below 40 K.or thema-bouts. The amountV of N2 left in the hydrogen Yis only small(eg. 0.003% at l2 ats.). The further reduction of the N2- ooncentrationbelow the maximum vapour tension at the distillation temperature desiredmay subsequently be effected with the aid of an adsorption filter. Atthis low temperature level these ltesrs possess a very high capacity,`so that it is possible to use filters of reason-able dimensions. Havingpassed through the filter, the hydro-` gen may-whether or not afterbeing expanded tothe distillation pressure-be cooled downto theiin-alternperature desired by leading it through a heat-exchanger in-countercurrent relation to the hydrogen obtained from the distillationprocess. The cold deficit occurring in the low temperature section as aresult of insulation losses etc. is madel up by the high-pressurehydrogen cycle by expanding said hydrogen.

After being cooled down in the regenerators, the highpressure hydrogenis preferably not expanded to the pressure of the hydrogen distillationbut, as before referred to, to a pressure corresponding with thepressure of the hydrogen supplied. In this case thepressure of thehighpressure hydrogen is so selected that the cold balance can be put inequilibrium. For from an energy-point of-vewit is-advantageous not toexpand Vthe high-pressure hydrogen any further; for example theexpansion of hydrogen from 100 to l2 ats. already yields the largerportion of the cold that can be obtained by expanding from l() to 1 at.,whilst in compressing a gas from 1.3 ats. to l2 ats.4 relatively muchmore energy is required than for compressing from l2 to 100 ats.- Sincethe ushing gas is now-V fed back at the same pressure at which it is tobe supplled to, say, the compressor of the ammonia plant, a considerableadvantage is obtained.

As already stated the invention is not restricted to the removal of N2from H2. The removal Ofother gases, such as argon and carbon monoxidemay be carried out in the same way. In practice, the latter puricationlmay even be carried out simultaneously with the former.

The process may also be used to advantage for cooling and purifyinggases other than hydrogen whilst def positing the impurities containedtherein. l

If the occurrence of pressure surges is not regarded as a seriousdisadvantage, it suiices to use one set of in dependent pipe systems perheat-regenerator.- Then, the compressed hot gas and the cold gas to becompressed may be-alternatel-y passed through said pipe system.

Instead of heat regenerators, reversing vheat-exchangers may also beusedas already referred to, provided their design satisfies the conditions`imposed by'rthe process. Foneigample the spaces .alternately'travesi bythe hybe packed with filling material of Va suiicientheat capacity;vThe, term reversing heat-exchanger wherey used in the following claimsmeans an exchanger satisfying the necessary conditions aforesaid.

It will be understood that the invention is not conned to theapplication of four regenerators. The same eiiect may also be obtainedwith two regenerators if' these are arranged in parallel with aheat-exchanger-or-heat-regenerator, in which the gases owingcountercurrently to the gas to be purified andv to the flushing gas arebrought in heat-exchange relation with each other. In that case, too,the` variation in the gas-quantities may be brought about in a simplemanner by distributing the said quantities over one of the twoheat-regenerators and theheat-exchanger arranged in parallel therewith.

I claim:

l'. A process for the purification of a gas by a heat exchangingregenerator system comprising cooling and depositing the impurities insaid gas by condensation to the solid state in a plurality of zones andin a plurality ofv stages, cooling the gas to be purified as it ispassed through a 'drst zone in countercurrent ow and in heat exchangingrelationwitha cold gas and then passing a flushing gas current throughsaid first zone in a, subsequent stage of the `process in a reversedirection and countercurrently and in heat exchanging relation with awarm gas to thereby evaporate and take up the deposited impurity, thetemperature of the gas to be puried and the warm gas owingcountercurrently with Ythe flushing gas at one end and the temperatureof the-ushing: gasand the cold gas flowing countercurrently with the gasto be purified atthe other end on entering the regenerator arerespectively substantially equal, controlling-the temperature in saidrstzone so that f thetemperature at any place in `said iirstzone is onthev average lower during. the period of depositionthan the averagetemperature in said first zone during the period vof evaporationwhilethe temperature on the momentof switching oftl1e gases is practicallythe same and creating; this temperature` differential by varying 'theamount of atleastone pair ofgases that are in heat exchanging-relationwith each other during the passage of said gases through said zones.

2. Process according to claim-1, wherein-.the pressure under-Which theflushing gas is passed through .thesaid first zone isl keptsubstantially equal-to theA pressure under which the gas tobepuriedApasses therethrough.

3. Process according ltofclaim- 1, wherein the temperature isV`controlledsothat the temperature at anyY given point of the`saidutirst zoneI is .substantially the vsame at the vcommencement Vasat the termination ofthe deposition and the ushing stage.

4. Process according to .claim 1, wherein during the deposition stage ineach zone the amount of ,the colder gas passingpin countercurrent andheat Vexchanging relation to the gas tobe cooled and purified isiirstincreased to a largerand subsequently decreased toA a smaller amount`than the averagefamount passed through during decroases andsubsequently increases.

V5. -Processaccording to claim l, wherein during theY iushng stage ineach zone the-amount of the warmer gas.

passing in countercurrent and heat exchanging relation to the fiushinggas is first increased to a larger and subsequently decreased to asmaler amount than the average amount passed through during the wholefiushing stage and so that during this stage the temperature at anygiven point of the zone first increases and subsequently decreases.

6. Process according to claim 1 wherein four zones are employed, andwherein the puriiication is carried out continuously in a cycle of fourperiods in each of which cooling with deposition of impurity takes placein two of the zones which are temporarily connected in parallel with thesupply of the gas to be purified, and ushing takes place in the othertwo zones which are temporarily connected in parallel with a supply offlushing gas, the parallel pairing of the zones changing from period toperiod but so that there are two consecutive deposition periods and twoconsecutive flushing periods in each cycle for each zone.

7. Process according to claim 6, wherein each zone has at least twoindependent conduit systems through each of which gas can be conductedin countercurrent and heat-exchange relation with one of (a) the gas tobe purified and (b) the flushing gas owing through the said zone andwhere the separate conduit systems of each zone are connected into twoindependent systems common to the four zones, one associated with asource of supply of a gas colder than the gas to be purified and theother with a gas warmer than the flushing gas and wherein the flow fromsuch sources of supply during the four periods is directed so that ineach zone the flow of gas to be cooled and purified always takes placein countercurrent and heat exchanging relation with a flow of the coldergas and the flow of flushing gas always takes place in countercurrentand heat exchanging relation with a flow of the warmer gas.

8. Process according to claim 7 wherein the temperature at any givenpoint of each zone is caused regularly to lluctuate during at least oneof (a) the deposition periods and (b) the flushing periods bycontrolling the amount of gas forming respective currents which pass inheat exchanging relation during said deposition and iiushing periods, sothat at said given point the temperature at the end of the firstflushing period is higher than at the end of the first depositionperiod.

9. Process according to claim 8, wherein the said amounts of gas arecontrolled so that at any given point in each zone the temperature atthe end of the first deposition period is lower than at the end of thesecond deposition period and at the end of the second ushing period.

10. Process according to claim 8, wherein the said amounts of gas arecontrolled so that at any given point in each zone, the temperature atthe end of the first ushing period is higher than at the end of thesecond deposition period and at the end of the second iushing period.

11. Process according to claim 1 wherein in the zones, the flow of gasto be cooled and purified always takes place in countercurrent and heatexchanging relation with a iiow of colder gas and the ow of flushing gasalways takes place in countercurrent and heat exchanging relation with aow of warmer gas, and wherein the said colder gas is gas which haspreviously been purified and which is at a lower pressure than the gasto be cooled and purified passing through the zone and wherein aftertraversing the zone the said colder purified gas, after continuedheating, is compressed to a pressure higher than that of the gas to bepurified, and after having cooled down to the temperature levelprevailing at the warmer end of the zone, is, at least in part, passedthrough the zone at the said higher pressure, as the said warmer gas, incountercurrent with flushing gas, whereafter, the said gas at the higherpressure is expanded, simultaneously delivering its cold, substantiallyto the pressure of the gas to be puried and then returned as fiushinggas through the zones.

12. Process according to claim 1 applied in the purification ofhydrogen.

13. Process according to claim 12, applied for the removal of residualnitrogen from hydrogen.

14. Process according to claim 13, carried out in the course of thedistillation of hydrogen.

15. In a process for distilling hydrogen and recovering the deuteriumcontained therein, the improvement comprising cooling and purifying thehydrogen to the distillation temperature by the process of claim 11.

16. An apparatus suitable for the purification of a gas by cooling anddeposition of solids therefrom comprising four heat exchange meansoperatively connected together, each heat exchange means having apassage for gas to be cooled and purified and subsequently for flushinggas, said passage being operatively connected to a source of said gas tobe cooled and purified and also being operatively connected to a sourceof said flushing gas, each of said heat exchange means also having atleast two independent conduits through each of which gas can beconducted in countercurrent and heat exchanging relatoin with gasiiowing through said passage, said heat exchange means being connectedin such manner that during a cycle of four periods cooling withdeposition of impurity takes place in two of the heat exchange meansconnected in parallel while flushing takes place in the other twoexchange means also connected in parallel, means for changing theparallel pairing of the heat exchange means during the four-period cycleso that during each cycle there are two consecutive deposition periodsand two consecutive fiushing periods in each heat exchange means, saidindependent conduits being parts of two independent systems common tothe four heat exchange means, one of the independent systems enabling agas warmer than the flushing gas to be passed in any period through theheat exchange means then being fiushed and the other of the independentsystems enabling a gas colder than the gas being purified to be passedin the heat exchange means in which deposition is taking place and meansfor controlling the amounts of gas passing through two of the heatexchange means in parallel currents from at least one source of supplygas to render the two parallel currents unequal.

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